U.S. patent number 6,611,338 [Application Number 09/713,076] was granted by the patent office on 2003-08-26 for method and apparatus for the interferometric examination of scattering objects.
This patent grant is currently assigned to ISIS Optronics GmbH. Invention is credited to Alexander Knuttel, Christian Kugler, Reiner Rygiel, Paul Welker.
United States Patent |
6,611,338 |
Knuttel , et al. |
August 26, 2003 |
Method and apparatus for the interferometric examination of
scattering objects
Abstract
The invention proposes a method for the optimization of the
interferometric examination of scattering objects, wherein
intensity-modulated light is divided, one beam is directed into an
object and the other beam is directed to a reference mirror, the
reflected light is guided to a detector module, where it is
converted to an interference signal and this signal is evaluated.
The method is characterized by the fact that light of at least two
different central wavelengths is irradiated and the converted
interference signals of both central wavelengths are phase-shifted
in order to compensate for their expected dispersion.
Inventors: |
Knuttel; Alexander (Birkenau,
DE), Welker; Paul (Bissersheim, DE),
Kugler; Christian (Schwetzingen, DE), Rygiel;
Reiner (Dossenheim, DE) |
Assignee: |
ISIS Optronics GmbH
(DE)
|
Family
ID: |
7929333 |
Appl.
No.: |
09/713,076 |
Filed: |
November 15, 2000 |
Foreign Application Priority Data
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Nov 17, 1999 [DE] |
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199 55 268 |
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Current U.S.
Class: |
356/479 |
Current CPC
Class: |
G01B
11/2441 (20130101); G01B 9/02007 (20130101); G01B
9/02091 (20130101); G01B 9/02014 (20130101); G01B
9/02083 (20130101) |
Current International
Class: |
G01B
11/24 (20060101); G01B 9/02 (20060101); G01B
009/02 () |
Field of
Search: |
;356/479,477,484,497 |
References Cited
[Referenced By]
U.S. Patent Documents
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5570182 |
October 1996 |
Nathel et al. |
5795295 |
August 1998 |
Hellmuth et al. |
5835215 |
November 1998 |
Toida et al. |
6015969 |
January 2000 |
Nathel et al. |
|
Foreign Patent Documents
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44 29 578 |
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Jun 1996 |
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DE |
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197 00 592 |
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Jan 1997 |
|
DE |
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WO 92/19930 |
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Nov 1992 |
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WO |
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WO 97/27468 |
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Jul 1997 |
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WO |
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Primary Examiner: Turner; Samuel A.
Assistant Examiner: Lyons; Michael A.
Attorney, Agent or Firm: Hackler; Walter A.
Claims
What is claimed is:
1. Apparatus for the interferometric examination of a scattering
object with an optical coupler, a reference mirror and a detector
module in the light path, the apparatus comprising: at least two
light sources (2, 2') with different central wavelengths
(.lambda.1, .lambda.2); a digitalization unit (18) connected to the
detector module (15), for digitalization of received, converted
interference signals of light of the at least two light sources (2,
2'); and a shifting unit (20) for shifting a phase position of the
central wavelength (.lambda.1, .lambda.2) in order to compensate
expected dispersion of the phase position.
2. Apparatus according to claim 1, having a difference formation
unit serving for the formation of a magnitude difference between
the added signals (in-phase-signal, I.sub.in) and the subtracted
signals (out-off-phase-signal I.sub.out) from the two central
wavelengths (.lambda.1, .lambda.2), according to:
wherein W is a weighting factor.
3. Apparatus according to claim 1, comprising a phase and velocity
determination unit (22, 23), in order to determine velocities
V.sub.p in the object, according to
wherein .lambda. is one of the central wavelengths, V.sub.o is the
velocity of the reference mirror and
.DELTA..phi.=.phi.(z.sub.1)-.phi.(z.sub.2) is the phase shift.
4. Apparatus according to claim 1, wherein the light sources are
designed as a one- or two-dimensional array.
5. Apparatus according to claim 1, wherein the light sources are
designed as surface light sources.
6. Apparatus according to claim 5, wherein surface light sources
are surface emitting LED's.
7. Apparatus according to claim 1, wherein the detector is designed
as an one- or two-dimensional array or as a CCD detector.
8. A method for interferometric examination of a scattering object
using an interferometer having a light source, a sample arm, a
reference arm and a detector arm, the method comprising the steps
of: dividing intensity-modulated light of at least two different
central wavelengths into a first partial beam, traveling along the
sample arm, and a second partial beam, traveling along the
reference arm; irradiating the scattering object with the first
partial beam to produce scattered light; directing the second
partial beam to a reference mirror to produce a reflected second
partial beam; combining at least some of the scattered light and at
least some light of the reflected second partial beam along the
detector arm to a detector module; opti-electronically converting
the combined scattered light and light of the reflected second
partial beam in the detection module into at least two interference
signals with different central wavelength; and shifting the
converted interference signals relative to each in order to
compensate expected dispersion of phase position.
9. The method according to claim 8 further comprising the step of
digitizing and storing the converted interference signals before
shifting the converted interference signals relative to each
other.
10. The method according to claim 9 wherein the at least two
different central wavelengths of the intensity-modulated light have
a phase shift of approximately .pi./2.
11. The method according to claim 8 wherein the intensity modulated
light is modulated with continuous intensity modification.
12. The method according to claim 11 wherein the intensity
modulated light is modulated sinusoidally.
13. The method according to claim 8 wherein the intensity modulated
light is modulated digitally.
14. The method according to claim 8 further comprising the step of
adding the at least two interference signals to form an in-phase
signal, I.sub.in, subtracting the at least two interference signals
from one another to form an out-of-phase signal, I.sub.out, and
determining a difference of magnitudes of the in-phase signal,
I.sub.in, and the out-of-phase-signal, I.sub.out, using a weighting
factor W according to
15. The method according to claim 8 wherein signal magnitudes of
the interference signals are formed and added incoherently.
16. The method according to claim 8 further comprising the step of
moving the reference mirror with a velocity in order to obtain
depth sampling of the scattering object.
17. Apparatus for the interferometric examination of a scattering
object, the apparatus comprising: at least two light sources (2,
2') with different central wavelengths (.lambda.1, .lambda.2); an
optical combiner (4) for combining light from each of the at least
two light sources into an optical waveguide (5); an optical coupler
(6) for dividing the combined light into a sample arm (7) and a
reference arm (8), said sample arm (7) being disposed for
irradiating the scattering object with a first partial beam of the
divided light and receiving scattered light, said referenced arm
(8) being disposed for irradiating a mirror (12) and receiving
reflected light; a light detector (14) for receiving both scattered
and reflected light; a digitalization unit (18) connected to said
light detector (14) for digitalization of received converted
interference signals between the scattered and reflected light from
the two light sources; and a shifting unit (20) for shifting a
phase position of the centered wavelengths (.lambda.1, .lambda.2)
in order to compensate expected dispersion of the phase position.
Description
BACKGROUND OF THE INVENTION
The invention applies to a method and an apparatus for the
interferometric examination of scattering objects.
Scattering objects, in particular strongly scattering materials,
can be investigated with optical coherence tomography (OCT),
according to the current state of the art up to a penetration depth
in beam direction of 3 mm. Together with a sampling, transversal to
the beam direction, in one or in both dimensions, a 2D- or
3D-picture is generated.
During the interferometric sampling, a partial beam reflected by
the object interferes with a partial beam reflected by a reference
mirror which is movable in beam direction. Alternatively, the
entire interferometer can be moved. The interference signals,
converted opto-electronically by sensors, are demodulated to form a
unipolar demodulation signal, used to obtain a digital image of the
object. Envelope curves of the interference signals result. With
respect to the current state of the art, reference is made to WO
97/27468 the contents of which is incorporated herein by
reference.
The more broadband the irradiated light is, the smaller is the full
width half maximum (FWHM) of the demodulation signal and thus the
spatial resolution in depth direction. This, however, is impaired
by dispersion due to wavelength-depending transit time differences
of the received signals between object arm and reference arm. It is
tried to achieve a dispersion compensation by balancing both
optical arms. However, due to the unknown optical characteristics
of the object to be investigated, which can even change depending
of the penetration depth, this cannot be completely obtained. A
complete compensation is hardly possible as both optical arms
cannot be exactly the same. This becomes even more relevant if
higher orders of dispersion have to be taken into account.
Furthermore, additional elements used for compensation can cause
negative results concerning the signal propagation in the
interferometer.
In the field of medicine, optical coherence tomography can be
applied for the examination of scattering objects accessible
outside the body, as e.g. skin, nails, lips etc., or
endoscopically, for the examination of the bronchial system, the
gastrointestinal tract, or the lungs. In non-medicinal fields, the
examination of thin, optically scattering plastic or ceramic layers
is possible.
It has already been proposed to increase the spectral bandwidth by
combining a plurality of central wavelengths, in order to increase
the depth resolution. This is known for example from U.S. Pat. No.
5,795,295. Further interferometer arrangements working with two
central wavelengths are known from U.S. Pat. No. 5,835,215, DE
19700592 A1 and WO 92/19930 A1. From DE 4429578 A1, use of
intensity modulated light is known.
BRIEF SUMMARY OF THE INVENTION
The invention addresses the problem of making available a method
and a device for further increasing the spatial resolution in
optical coherence tomography.
According to the invention, this problem is solved by a method for
the interferometric examination of scattering objects, wherein
intensity-modulated light is divided, one beam directed into an
object and another beam directed to a reference mirror, the
reflected light is lead to a detector module by which it is
converted opto-electronically to an interference signal and this
signal is evaluated, light of at least two different central
wavelengths is irradiated into the object and onto the reference
reflector and the converted interference signals of the central
wavelengths are shifted relative to each other for compensating the
expected dispersion of their phase position.
The intensity modulation of the irradiated light allows a
considerable increase of the instantaneous maximum light intensity
as compared to a continuous light beam, without an increase of the
overall power and without an unfavorable increase of the device
temperature and the object temperature. This also increases the
detected interference signal, thus obtaining an improved
signal-to-noise ratio. If a fast A/D-converter is used, e.g. 10
MHz, and for modulation frequencies of the interference signal in
the 100 kHz range, an On/Off sampling ratio of 1/100 is obtained.
The maximum light power can be increased in the reciprocal ratio
(here: 100/1), without increasing the average overall power. This
is particularly true for surface emitters, as e.g. surface emitting
LED's.
The intensity modulation of the signals of the at least two central
wavelengths, is preferably phase-shifted relative to each other.
The detected interference signals are preferably digitized and
stored in a computer. Thereafter the saved digital values of both
interference signals are shifted against each other in the
computer, for dispersion compensation. The intensity modulation of
the irradiated light, respectively of one wavelength, is preferably
realized with a phase shift of about .pi./2 of the respective
central wavelength. An exact offset of .pi./2 for different central
wavelengths, however, cannot be obtained, and is not necessary.
According to preferred embodiments of the invention, a digital
modulation of the light from the light source (by switching on and
off), or a modulation with continuous intensity change may be used.
In the latter case, the light is modulated, in particular, with a
sinus wave. A disadvantage of this soft intensity modulation with
sinosoidal control is a decrease of the effective sampling ratio,
and thus, a decrease of the effective power gain. On the other
hand, there are no high frequency components due to sharp pulse
edges, as with digital modulation.
The invention generates an effective spectrum which offers much
more bandwidth than the spectrum of the individual light sources.
However, the dispersion is limited to the individual dispersion of
the individual light sources (for one central wavelength). This
results in the following advantages: Reduced dependence of the
dispersion from the penetration depth of the light into the object;
good dispersion compensation also for higher orders; no additional
optical materials in one of the optical object arms or reference
arms.
For a further FWHM reduction, a preferred embodiment of the
invention proposes the formation of the magnitude difference
between the added signals (in-phase-signal, I.sub.in) and the
subtracted signals (out-off-phase-signal I.sub.out) from at least
two interference signals with different central wavelengths, taking
into account a weighting factor W, according to the following
equation:
The weighting factor should preferably be <1 and >0.3. A
corresponding difference formation unit may be provided in the
computer of the apparatus of the invention.
A further embodiment comprises the formation of the signal
magnitudes of at least two central wavelengths, and the incoherent
addition of the magnitude signals.
As an interference signal can be divided into magnitude and phase,
another advantageous variant of the method of the invention is
characterized by that fact that the particle velocity (v.sub.p) in
the object is determined relative to the (known) travel velocity
v.sub.o of the reference mirror, deducted from the phase difference
.DELTA..phi.=.phi.(z.sub.1)-.phi.(z.sub.2) according to
wherein .DELTA.Z=z.sub.1 -z.sub.2.
The apparatus is equipped with phase and velocity determination
units, for determining velocities v.sub.p in the object, according
to this method.
BRIEF DESCRIPTION OF THE DRAWINGS
The light sources may be embodied as one- or two-dimensional array
light sources, respectively, or as surface light sources, as e.g.
surface emitting LED's. In analogy to the light source, the
detector may be an individual element, a one- or two-dimensional
array, or a CCD detector.
Further advantages and characteristics of this invention can be
taken from the claims and from the subsequent description of
preferred embodiments of the invention, referring to the drawings,
wherein:
FIG. 1 shows the design of an apparatus according to the invention
with optical waveguide system,
FIG. 2a shows the central range of an interference signal of two
different central wavelengths;
FIG. 2b shows the marginal range of the interference signal of the
two different central wavelengths of FIG. 2a;
FIGS. 3a-d show different possibilities of intensity modulation in
the scope of the method according to the invention;
FIGS. 4a-d show demodulated signals with two light beam
wavelengths, without dispersion--theoretically-- with a dispersion
of 3.3 ps/nm.multidot.km with a dispersion of 6.6 ps/nm.multidot.km
also with a dispersion of 6.6 ps/nm.multidot.km, but with a shift
of the interference signals, according to the invention;
FIGS. 5a-c show the signals of FIGS. 4a, 4b and 4d, after further
correction by formation of the difference between added and
subtracted interference signals; and
FIGS. 6a-d show diagrams related to the influence of the dispersion
onto the phase trajectory, and for the determination of a travel
velocity in the object to be examined, based upon the phase
positions in the interference signal.
DETAILED DESCRIPTION
FIG. 1 shows a first design of an apparatus according to the
present invention. It is equipped with a control unit 1 for the
control of two light sources 2, 2', with different central
wavelengths .lambda.1, .lambda.2 preferably in the infrared range.
Light sources 2, 2, 2' may be, e.g., light-emitting diodes (LED).
Light sources 2, 2, 2' are connected to optical waveguides 3, 3'
leading to a beam combiner 4; a further optical waveguide 5 leads
to an optical coupler 6. This coupler divides the light from the
light source arm 5 to the sample arm 7 and to the reference arm 8.
Via the sample arm 7 and an optical system 9 the light is
irradiated into an object O to be examined. Via the reference arm
8, and an optical system 11 next in line, the light is directed to
a reference reflector 12, movable in beam direction (vertical to
its surface), allowing a (continuous) change of the optical path
length between beam splitter 6 and reference reflector 12. The
relative change of the sample arm relative to the reference arm
which is required for the depth sampling of the object, is obtained
either by moving the reference reflector or the object.
Alternatively, an interferometer which has a static reference arm
can be moved, relative to the object.
The light scattered back from the object O and reflected by the
reference reflector 12, enters into the sample arm 7 and into the
reference arm 8, and is lead, via coupler 6, into a light guide
adjacent to the light source arm 5, forming the detector arm 13 of
the apparatus. The light of both wavelengths is received by the
light detector 14, which is located at the end of detector arm 13.
After an opto-electronic conversion, the received signals are lead
to an electronic processing unit 17 which comprises a
digitalization unit 18, a memory register 19, a shifting unit 20,
and, if necessary, a difference formation unit 21. Furthermore, the
processing unit 17 is equipped, downstream from the memory register
19, with a phase determination unit 22 and, further downstream,
with a velocity determination unit 23. The electronic processing
unit may be provided with output devices, e.g. a monitor 24, or a
printer 25. In the digitalization unit 18, the analogous
interference signals are digitized by sampling them in given
intervals and storing the obtained values, according to the
sequence shown in FIG. 3, in successive storage cells; the entirety
of these values represents the demodulation signal. Movements in
the object can be detected by the phase and velocity determination
units 22, 23.
The control unit 1 is equipped with a modulation device 1a for
phase-shifted modulation of the light sources 2, 2', preferably
realized by time-shifted pulsing of the light sources 2, 2'. The
modulation device 1a is synchronized with the digitalization unit
18 via connection 21.
The optical elements 3 to 13 may be in form of an integrated
optical circuit (optical chip), as known from WO 97/27468.
Furthermore, the light sources 2, 2' and the detector 14 may be
integrated therewith or coupled thereto.
Instead of an optical arrangement 2 to 14 operating with optical
waveguides or integrated in an optical chip, it is also possible to
realize an optical free beam arrangement by means of mirrors; in
this case, suitable optical elements, e.g. lenses, are arranged
downstream from the light sources 2, 2'. The beam combiner 4 and
the optical coupler 6 can be designed as beam splitters by means of
semitransparent mirrors.
A combination of an integrated optical system with optical
waveguides and a free-beam optical system is possible, too.
Whereas the light sources 2, 2' are shown as spatially separated
light sources in FIG. 1, light sources of different light
wavelength may also be integrated in a compact emitter.
FIGS. 2a and 2b show an interference signal 31, generated by a
apparatus according to FIG. 1, by exposure of an object O to light
with different central wavelengths and converted by detector 14, in
its central range in FIG. 2a and in its marginal range in FIG. 2b.
The envelope curves represented by the rhombs, stand for the
corresponding demodulation signal/magnitude signal. The dotted and
dashed curves represent the signals 32, 33, caused by different
irradiated wavelengths, in case of a continuous, unpulsed
irradiation. The closed points and the circles (not filled in)
represent sampling points for the digitalization of the received
signal, namely the open circles for wave 32 and the closed points
for wave 33. The points in time at which pulsed light is irradiated
are with respect to both wavelengths synchronous to these detection
points. For the central peaks of both signals (superposed here,
whereas divergent in the marginal range; FIG. 2b)it is clearly
visible that the phase difference between two irradiated pulses is
approximately .pi./2.
The central wavelengths alternate between the irradiated pulse
pairs. For the curve in FIG. 2, two pulses are irradiated with a
distance of approximately .pi./2: First for one central wavelength
(curve 32) and then for the other central wavelength (curve 33). As
the pulse trains of both wavelengths have a non-constant phase
relation to each other, their phase positions are shifted over the
depth Z of the interference signal. Furthermore, from FIGS. 2a and
2b it can be seen that in the central range (FIG. 2a) of the
interference signal, the carrier signals are practically in phase,
whereas in the marginal range (FIG. 2b)the signals are out of phase
by .pi..
FIG. 3 shows different alternatives for pulsed irradiation of two
different central wavelengths. In FIG. 3a, the same wavelength is
irradiated subsequently two times, i.e. the first wavelength and
the second wavelength at phase 0 and approximately .pi./2
respectively.
For a phase difference of approximately .pi./2, the demodulation
signal can be determined in known manner. Even if the phase
difference differs from .pi./2 (e.g. in the range of .pi./4 to
3.pi./4) the demodulation signal can be calculated, if the phase
difference is known. However, the bigger the difference from .pi./2
is, the bigger is the scattering of the calculated values. FIG. 3b
shows alternating irradiation of central wavelengths before cycle
repetition.
The distance of the pulses of identical central wavelengths can
amount to an odd multiple of .pi./2. In the example of FIG. 3c,
three phase positions were chosen: 0, .pi./3 and 2.pi./3. In FIG.
3d, it is supposed that the pulses (with different central
wavelengths) are irradiated at such short intervals that the
detection unit almost "sees" the same phase situation. The
statements made with reference to FIGS. 3a and 3b are valid for the
demodulation of the signals. Any combination of the cases 3a-3d is
conceivable.
The method of the invention comprises a compensation of the
dispersion of two or more different irradiated wavelengths. This is
accomplished by shifting the received and converted interference
signals against each other, by the spatial shift expected due to
their dispersion D. The shift .DELTA.S amounts to:
with c=speed of light, z.sub.o =total light path. After the
digitalization of the interference signals, the shift is reflected
in a corresponding storage cell difference of the values which
represent the demodulation signals of the two central wavelengths.
Thus, a corresponding shift of the saved signals or storage cells,
respectively, can take place in order to correct the
dispersion.
FIG. 4a shows a theoretical demodulated signal, obtained by
calculation, without a dispersion, for central wavelengths
.lambda.1 of 1275 nm and .lambda.2 of 1325 nm, thus, for a
difference of 50 nm. For an individual bandwidth of both central
wavelengths of .DELTA..lambda.=60 nm, a mean full width half
maximum of 8.64 .mu.m results.
The influence of the dispersion of the individual bandwidth of a
light source is considered as negligible if the phase shift caused
by dispersion is about .pi./4. This means that the dispersion
(1.sup.st order) must comply with the following criterion:
##EQU1##
The following example shall explain the facts: For a central
wavelength of .lambda..sub.n =1300 nm, an individual bandwidth of
.DELTA..lambda.=60 nm and a supposed geometrical length (forward
and backward travel of the light) of the object arm and the
reference arm, respectively, of 2z.sub.o =120 mm, a dispersion of
.vertline.D.vertline..ltoreq.3.3 ps/(km.multidot.nm) results (for
light speed of c=3.0 mm/ps). Such a dispersion value can in
practice be obtained by a very symmetrical design of the optical
arms, or by a compensation with corresponding optical
materials.
However, from the square dependence of the dispersion with respect
to the individual bandwidth stated in equation (4) above, it is
readily apparent that a bandwidth increase of the individual light
sources to values of more than 60 nm, quickly exceeds the limits
set above, so that dispersion significantly deteriorates the
measurement signal quality. For example, a bandwidth of
approximately 100 nm results in a dispersion of approximately 6.6
ps/(km.multidot.nm). This results in a mean full width half maximum
of 12.3 .mu.m, leading to a deterioration of the spatial resolution
of 42%.
For this dispersion of 6.6 ps/(nm.multidot.km), all other values
being the same, the interference signal shift according to the
invention allows the reduction of the mean full width half maximum
to 9.15 .mu.m, as shown in FIG. 4d. This value is only 11% worse
than the theoretical value of FIG. 4a.
Furthermore, according to a preferred embodiment of the invention
it is suggested that the magnitude difference between the added
signals (in-phase-signal, I.sub.n) and the subtracted signals
(out-off-phase-signal I.sub.out) of both central wavelengths is
determined according to the following equation:
wherein I.sub.ges is the resulting overall intensity and W is a
weighting factor.
FIG. 5 shows an example for a weighting factor W=0.5. It is true
that a lower dispersion of D=3.3 ps/(nm.multidot.km) slightly
deteriorates the FWHM of FIG. 5b as compared to a dispersion of
D=0; however, the height of the (interfering) side bands is
reduced. A deliberately introduced low dispersion is conceivable,
too. For an increased dispersion, as e.g. D=6.6 ps/(nm.multidot.km)
in FIG. 5c, the dispersion correction significantly improves the
FWHM of the principal maximum.
The determination of movement speeds (equation (2)) causing a
Doppler effect in the material to be examined, is represented in
FIG. 6. The figures show the phase in rad of a
dispersion-compensated interference signal as a function of the
position of the reference mirror.
FIG. 6a shows the phase as a function of the depth z. It contains
complementary information to the mere magnitude image in FIG. 4a.
The velocity of the dispersed particle is zero. The dispersion was
set to zero, too. In FIG. 6b, a dispersion of D=6.6
ps/(nm.multidot.km) is supposed (a complementary magnitude image is
shown in FIG. 4c). The phase curve at z=0 is non-linear, thus, a
linearized fit according to equation (2) is quite difficult. After
the compensation of the dispersion (FIG. 6c) the phase curve at z=0
is significantly linearized. As expected, the matched phase change
is zero.
A phase change at z=0, of .DELTA..phi./.DELTA.z=0.06
.pi./.mu.m--with the parameters corresponding to those of FIG.
4d--can be taken from FIG. 6d; thus, as to equation (2), for a mean
central wavelength of .lambda.=1.3 .mu.m a velocity ratio v.sub.p
/v.sub.o of 0.02, and, for a velocity of the depth scan of v.sub.o
=50 mm/sec, a particle velocity of 1 mm/sec results.
* * * * *